(See also Overview of Sunlight and Skin Damage.)
Photosensitivity, sometimes referred to as a sun allergy, is an immune system reaction that is triggered by sunlight. Photosensitivity reactions include solar urticaria, chemical photosensitization, and polymorphous light eruption and are usually characterized by an itchy eruption on patches of sun-exposed skin. People may inherit a tendency to develop these reactions. Certain diseases, such as systemic lupus erythematosus and some porphyrias, also may cause more serious skin reactions to sunlight.
Hives (large, itchy red bumps or welts) that develop after only a few minutes of exposure to sunlight are called solar urticaria. The hives typically last for minutes or hours. This disorder can be difficult to treat, but doctors may prescribe histamine (H1) blockers, antimalarial drugs, corticosteroids, sunscreens, or ultraviolet (UV) light therapy. A person can be prone to developing solar urticaria for a very long time, sometimes indefinitely. People with large affected areas sometimes have headaches and wheezing and feel dizzy, weak, and nauseated.
Over 100 substances, swallowed or applied to the skin, are known to cause sun-induced reactions on the skin. A limited number cause most reactions (see Table: Some Substances That Sensitize the Skin to Sunlight). To treat chemical photosensitivity reactions, corticosteroids are applied to the skin and the substance that is causing the reaction is avoided. There are two types of chemical photosensitivity: phototoxicity and photoallergy.
In phototoxicity, people have pain and develop redness, inflammation, and sometimes brown or blue-gray discoloration in areas of skin that have been exposed to sunlight for a brief period. These symptoms resemble those of sunburn, but the reaction differs from sunburn in that it occurs only after the person has swallowed certain drugs (such as tetracyclines or diuretics) or chemical compounds or has applied them to the skin (such as perfume and coal tar). Some plants (including limes, celery, and parsley) contain compounds called furocoumarins that make some people's skin more sensitive to the effects of UV light. This reaction is called phytophotodermatitis. All phototoxic reactions appear only on areas of skin that have been exposed to the sun. They usually develop within hours after sun exposure.
In photoallergy, an allergic reaction causes redness, scaling, itching, and sometimes blisters and spots that resemble hives. This type of reaction can be caused by aftershave lotions, sunscreens, and sulfonamides. Substances that cause photoallergy are capable of doing so only after the person has been exposed to both the substance and sunlight (because sunlight is what makes the substance capable of triggering photoallergy). Photoallergic reactions can affect areas of skin that have not been exposed to the sun. They usually develop 24 to 72 hours after sun exposure.
This eruption is a reaction to sunlight (primarily UVA light) that is not fully understood. It is one of the most common sun-related skin problems and is most common among women and among people from northern climates who are not regularly exposed to the sun. The eruption appears as multiple red bumps and irregular, red, raised areas (called plaques) and, rarely, as blisters on sun-exposed skin. These plaques, which are itchy, typically appear 30 minutes to several hours after sun exposure. However, new patches may develop many hours or several days later. The eruption usually goes away within several days to weeks. Typically, people with this condition who continue to go out in the sun gradually become less sensitive to the effects of sunlight (a process known as hardening).
There are no specific tests for photosensitivity reactions. A doctor suspects a photosensitivity reaction when a rash appears only in areas exposed to sunlight. A close review of the person's medical history, skin symptoms, any diseases, drugs taken by mouth, or substances applied to the skin (such as drugs or cosmetics) may help a doctor pinpoint the cause of the photosensitivity reaction. Doctors may do tests to rule out diseases that are known to make some people susceptible to such reactions (such as systemic lupus erythematosus).
When a rash occurs on an area of skin that has been exposed to the sun and the diagnosis is not clear, doctors may do skin patch tests and reaction reproduction tests that involve exposure to UV light (phototesting) when the person is not using any drugs that cause photosensitivity reactions. These tests may help clarify which type of photosensitivity reaction may be the cause.
All people should avoid excessive sun exposure, but people who are sensitive to sunlight due to any cause should be especially careful and wear protective clothes, avoid sunlight as much as possible, and use sunscreens regularly. If possible, any drugs or chemicals that could cause photosensitivity should be discontinued after consulting with a doctor.
People with polymorphous light eruption or photosensitivity caused by systemic lupus erythematosus should be seen by a doctor and sometimes benefit from treatment with corticosteroids applied to the skin or hydroxychloroquine or corticosteroids taken by mouth. Occasionally, people with this problem can be desensitized to the effects of sunlight by gradually increasing their exposure to UV light (see Phototherapy: Using Ultraviolet Light to Treat Skin Disorders).